1. Field of the Invention
The present invention relates to a sheet supplying apparatus and a reading apparatus using such a sheet supplying apparatus. More particularly, it relates to a reading apparatus such as a facsimile system for separating originals one by one and for reading the separated original. However, the present invention is not limited to the original supplying apparatus, but may be applied to a copy sheet supplying apparatus, for example. The sheet including the original, copy sheet and the like is generically referred as "sheet" hereinafter.
2. Related Background Art
In order to separate and supply stacked sheets one by one, there has been proposed a method for supplying a sheet one by one from an uppermost sheet and a method for supplying a sheet one by one from a lowermost sheet. For example, in copying machines, recording sheets are supplied one by one from an uppermost sheet; whereas, in automatic original supplying apparatuses for copying machines and facsimile systems, originals are supplied one by one sequentially from a lowermost original.
In the copying machines, generally, copied recording sheets (copy sheets) are discharged with imaged surfaces facing upside and are stacked in a reverse page sequence (i.e. from the last page toward the first page; namely, the lastly discharged recording sheet is the first page). Thus, the originals to be read are supplied from the last page. In this case, the originals stacked on an original stacking plate with imaged surface facing upside are supplied one by one from the lowermost one and the supplied original is reversely rotated (turned over). After reading, the originals are returned onto the original stacking plate while turning over the originals.
On the other hand, in the facsimile systems, since it is practical that the imaged sheets are successively transmitted to the receiver from the first page, the stacked originals are successively read in the page sequence (from the first page toward the last page). Further, in almost all of the facsimile systems, to prevent the system from becoming bulky, the original turn-over mechanism is omitted, and the originals are supplied with the imaged surfaces facing downside and are read in the page sequence, and the read originals are discharged in the page sequence.
In consideration of such background, it is practical that "the originals in the copying machine are stacked with the imaged surfaces facing upside and are supplied from the lowermost one" and "the originals in the facsimile systems are stacked with the imaged surface facing downside and are supplied from the lowermost one". Further, in order to permit the handling of a large number of originals, when the number of the originals is great, a conveying force of an original conveying roller is increased in accordance with a thickness of a sheet stack by displacing or elastically deforming an urging spring. For example, although the original is conveyed by a feed roller urged against a lower surface of the original stack or the conveying force is applied to the original by urging a pressure plate against an upper surface of the original stack, in any case, the conveying force depends upon the thickness of the original stack.
However, in the above-mentioned examples, since the original conveying force is adjusted only on the basis of the thickness of the original stack, even if the size of the original is changed, so long as the thickness of the original stack is unchanged, the conveying force cannot be adjusted in accordance with the changed original size. Accordingly, when the originals having large size are to be supplied in the same apparatus, an original separation portion must be designed to provide a greater conveying force. However, if the conveying force is increased to permit the supply of the originals having the large size, when the originals having small size are supplied, since the conveying force is too excessive, the double-feed of sheets and/or sheet jam will frequently occur. Thus, in all of the original supplying apparatuses, the number of the sheets which can be handled differs from each other depending upon the size of the original.